Today's high power SPSs are dominated by full-bridge and half-bridge SPSs. The full-bridge SPS comprises an input capacitor and two pairs of power switches. Each pair is connected across the capacitor. The primary winding of the power transformer is connected between common nodes of each pair. One switch of each pair is turned on so that the input voltage is applied across the primary winding. By switching all four switches, the polarity of the primary voltage is alternated.
The half-bridge SPS is also often used in a medium power range. The input voltage is applied to one pair of power switches. It is also usually applied to a pair of capacitors connected in series and thus constituting a voltage divider. The primary winding of the power transformer is connected between common nodes of each pair. The switches open and close in a sequence so that the capacitor voltages of opposite polarities and equal to one half the input voltage are alternately applied across the primary winding. An off-line SPS may comprise a pair of half-wave rectifiers so that the capacitors are charged to the positive and negative peaks of the line voltage.
The half-bridge SPS can also operate with a single input capacitor. The imbalance in operating flux level is corrected by an AC capacitor, which is connected in series with the primary winding. The AC capacitor must sustain peak currents of the power switches and the switching frequency of the SPS. The AC capacitor is charged to one half the input voltage. However, considering abnormal conditions such as a start-up of the SPS, the AC capacitor must sustain the input voltage.
The push-pull SPS comprises a single input capacitor and a pair of power switches. The primary winding of the power transformer has a center tap with the input voltage applied thereto. The switches are grounded and further separately connected to the primary ends. The switches open and close in a sequence so that the input voltage is alternately applied across each half of the primary winding. The winding constitutes a voltage doubler. When one switch is turned on, the voltage across the other switch is twice the input voltage.
Generally, resonant type SPSs are inherently inferior to square wave type SPSs. In the resonant type SPSs a sinusoidal voltage and/or current is developed. That, however, is associated with a considerable interval, usually near zero crossing of the respective sinusoidal signal, while a minimal or no energy transfer takes place. Therefore, the resonant SPSs require power semiconductors having higher current and/or voltage ratings. An output capacitor carries a high ripple current. By contrast, in square wave type SPSs a maximum energy transfer is accomplished instantaneously.
The resonant type SPSs have other inherent flaws. The resonant or switching frequency is determined by additional LC components, or an additional capacitor and the leakage inductance of the power transformer. These components are fixed and so is the turn-on or turn-off time. Pulse frequency modulation (PFM) rather than pulse width modulation (PWM) or other preferred switching method is often used to accomplish the regulation. A minimum load may be necessary to avoid large variations of the switching frequency. Moreover, the maximum switching frequency can be quite high if it is determined by the leakage inductance. The resonant type SPSs are preferred in special applications, such a high voltage conversion. General purpose SPS are costly and constitute a small fraction of all SPSs manufactured today.
The full-bridge, half-bridge and push-pull SPSs operating at the same input voltage and delivering the same output power can be compared. The full-bridge requires four switches to apply the input voltage across the primary winding and alternate the polarity. The half-bridge requires only two switches, whereas only one half of the input voltage is used. Consequently, currents flowing through the switches must be doubled to deliver the same output power. Moreover, the SPS requires two input capacitors or a single input capacitor and an AC capacitor carrying peak currents. The push-pull SPS applies the input voltage across separate halves of the primary winding. Only two switches have the same current ratings as four switches of the full-bridge SPS. That is also one half of the current ratings of the switches used in the half-bridge SPS.
The push-pull SPS has a lower efficiency than the full-bridge and half-bridge SPSs. When one switch of the full-bridge or half-bridge SPSs turns off, the leakage energy of the power transformer is returned to the input source. By contrast, the push-pull SPS dissipates the leakage energy in a passive snubber network. Furthermore, the switches have twice the voltage ratings. The push-pull SPS is therefore preferred when the input voltage is relatively low and/or the circuit simplicity is essential.
However, except for the above disadvantages, the push-pull SPS is inherently superior over the full-bridge and half-bridge SPSs. When one switch turns off, the input current of the full-bridge or half-bridge SPS responds in the worst possible manner. The leakage inductance of the power transformer causes the input current to change its polarity at the peak. The input capacitor or one of the series coupled input capacitors is then recharged by the current spike. This results in a voltage ripple that appears across the input of the SPS. Very often the current spike causes high frequency oscillations that are very difficult to filter. Furthermore, one or two floating power switch drivers are required. Different delay times of the floating and grounded drivers must be considered.
The input current of the push-pull SPS derives from the single capacitor and continues to flow when either switch turns off. The current is equal to the primary current that, at its peak, is replaced with the leakage current having the same amplitude and polarity. The input current is therefore unidirectional and continues to flow while being dissipated in the snubber. The primary current is conducted by only one switch and the respective half of the primary winding. Comparing with the full-bridge SPS, there are no conduction and switching losses of the additional switches. Comparing with the half-bridge SPS, there is no series coupled AC or DC capacitor and so there are no additional power losses caused by its equivalent series resistance and inductance (ESR and ESL). The switches of the push-pull SPS are grounded, which significantly simplifies a driver circuitry. The switches can be also connected to ground through a resistor for sensing the primary current and the short circuit current. Except for some noise, the voltage appearing across the resistor has one polarity.
Voltage-fed SPSs, including the full-bridge, half-bridge and push-pull SPSs referred to hereinabove, have numerous disadvantages. When the respective power switch turns off, the leakage energy stored in the power transformer is dumped directly to the input capacitor or snubber. The current drawn from the respective input capacitor reverses the polarity at its peak or, in best case, quickly drops from the peak to zero. The input current ripple reflects the secondary current of the transformer, has overlapping current transients and high frequency oscillations caused by the dumping of the leakage energy. The output inductor must carry the output current, which is usually one or two orders of magnitude larger than the input current of the SPS.
A dead time is necessary at each transition to allow the energized switch to fully turn off before the other switch turns on. However, the dead time decreases the efficiency as the SPS must switch at a duty cycle below 50%. To compensate for that, the input current must be increased, which causes higher conduction and switching losses. Moreover, during the dead time the output inductor pulls both output diodes into the conduction. This causes draining of the magnetic field from the power transformer. As the respective power switch turns on, it sees a virtual short circuit until the respective output diode is pulled out of the conduction.
A current-fed operation of the push-pull SPS is accomplished by connecting an input inductor in series with the center tap of the power transformer. The secondary voltage is rectified by a pair of diodes and applied directly to an output capacitor. The input inductor carries the input current that is usually one or two orders of magnitude smaller than the output current. Moreover, the inductor reduces an input current ripple and allows large input voltage transients. No dead time is necessary as the cross conduction current is limited by the inductor. No output inductor is required to filter the rectified output voltage of the power transformer and the related problem of the simultaneously conducting output diodes is eliminated. Furthermore, the voltage tracking between multiple outputs is significantly improved.
However, the duty cycle must remain above 50%. The switches are closed individually so that the inductor current is alternately applied to each half of the primary winding. When both switches conduct simultaneously the input inductor is charged, whereas no energy is transferred to the output. The SPS can operate at the duty cycle below 50% if an output inductor electromagnetically coupled to the input inductor is used. The output inductor and a series coupled additional diode are connected across the output capacitor. Minimum output load requirements are reduced and the output ripple voltage is minimized as the current applied to the output capacitor is continuous.
A preregulator can be also used while the push-pull section operates continuously at 50% duty cycle. The preregulator is usually a buck converter comprising an input switch, a catching diode and an output inductor. That inductor is connected in series with the center tap of the power transformer. However, this configuration has many problems. The input switch is floating and interrupts the input current. Peak currents of the input switch and catching diode can be significantly higher than an average input current. Finally, the push-pull section becomes merely an elaborate voltage amplifier having a fixed gain.
Power factor correction circuit simulates a resistive load. Line is most often used as the input source, wherein the input current is sinusoidal and matches the phase of the line voltage. A conventional power factor correction circuit employs a boost converter and operates as a preregulator of the SPS. The line voltage is rectified. A switch applies the rectified voltage across an inductor that carries the input current. Only when the switch is turned off, the energy is delivered to the input capacitor of the SPS itself. This results in peak input currents that are significantly larger than the expected input current. A sizable line filter is necessary to minimize the noise injected into the line. Moreover, the converter is coupled in series with the SPS and must sustain its full power.